Field
The present disclosure relates to image sensors having an anti-reflective/passivation layer and methods of manufacturing thereof.
Description of the Related Art
For applications involving imaging sensors and photo detectors, it is desirable to detect near-UV (near-ultraviolet) and visible as well as infrared wavelength radiation. The detectors may incorporate photodiodes and may be made of, for example, HgCdTe (mercury cadmium telluride, also referred to as MerCadTel or MCT). The detector may be, for example, a hybrid detector that incorporates infrared detector layers for detection of light and collection of photo charge into pixels and a silicon readout integrated circuit (ROIC) for converting the photo charge to voltage with an amplifier for each pixel. The ROIC may multiplex the signals from each pixel to off-chip electronics. An anti-reflective (AR) coating can be applied to reduce or prevent reflection of incident light, thereby increasing the quantum efficiency (QE).
FIG. 1A is an energy band diagram showing a prior art AR coating-detector assembly 100a having an AR coating layer 102a and a dielectric/passivation layer 104a being proximal to the back interface 126a. The dielectric/passivation layer 104a has a relatively narrow bandgap. It is desirable that when a photon with energy greater than the bandgap is absorbed, an electron be excited from the valence band and placed into the conduction band such that the photo charge generated in the valence band can be collected into pixels and measured. In order to achieve high QE on substrate-removed HgCdTe p/n photodiodes, a backside AR-coating layer 102a is applied to match the refractive index of HgCdTe to air/vacuum. The AR-coating layer 102a is formed by depositing transparent dielectric films onto the back n-type surface of the HgCdTe detector 130a. The AR-coating layer 102a can be deposited by, for example, evaporation or sputtering. When short-wavelength UV or visible radiation 120a is absorbed by the n-type HgCdTe (116a), energetic electrons are generated due to the incident radiation having energy significantly greater than the bandgap of HgCdTe. The excess kinetic energy of these electrons (e.g., electron 106a) that are generated in proximity to the back interface 126a allows them to be injected into the dielectric/passivation layer 104a or the portion of the AR-coating layer proximal to the back interface 126a, causing the dielectric/passivation layer 104a and/or the AR coating layer 102a to become negatively charged, as indicated by negative charging 128a. The charging can become progressively greater with increasing exposure duration.
The negative charging 128a in the portion of the AR-coating layer 102a that is most proximal to the back interface 126a and/or dielectric/passivation layer 104a pushes the majority carriers (e.g., electrons) away from the back interface 126a and attracts minority carriers (holes) (shown at energy level 122a), resulting in an inversion/depletion layer 108a. The bandgap is bent as shown in the shaded region 114a (which is positioned adjacent to the N—HgCdTe absorber layer 116a). The inversion/depletion layer or region 108a acts as a potential well that captures photo-generated holes 110a created when short-wavelength radiation is strongly absorbed at the back interface 126a. At cryogenic operating temperatures, the holes 110a do not have adequate thermal energy to surmount the barrier created by the potential well, preventing them from diffusing or drifting towards the front-side p-junction and P—HgCdTe 118a. As a result, short wavelength QE is reduced. The effect is most detrimental in the short wavelength since this radiation is absorbed and photo carriers are generated primarily within the extent of the backside potential well.